The present invention is directed towards a telephone apparatus, and more particularly, an apparatus for preventing and/or indicating to the user of a telephone that an attempt is being made by a third party to eavesdrop on his conversation.
With an increasing number of extension telephones and/or an increasing number of plug-in receptacles present in a residence or business, the problem of eavesdropping on telephone conversations is becoming more and more of a problem. The problem has been compounded by the increased use of private telephone equipment.
The prior art has recognized the problem of eavesdropping and has taken steps to reduce or eliminate the same by either alerting the user of attempts at eavesdropping or excluding a potential eavesdropper from audio access to telephone lines that are in use. For the most part, these prior art attempts have required complex circuitry and costly installations. U.S. Pat. No. 3,499,121 to MacIntosh et al. discloses a privacy circuit which ensures that only one telephone of a plurality of extension telephones connected to the same telephone line will gain access to the telephone line. In order to be effective, a respective privacy circuit must be provided for each extension telephone. The heart of the privacy circuit is an SCR which is connected in series with the extension telephone and a Zener diode which is connected to the gate of the SCR. The Zener diode is driven into reverse saturation when the extension telephone is first placed in the off-hook condition (defined herein as that condition wherein the hook switch of the telephone is in the upper or raised position) since the full line voltage (nominally 48 volts) will be available to the Zener diode. When driven into the reverse bias condition, the Zener diode will apply a firing signal to the gate of the SCR turning the SCR on. The SCR will stay on as long as driving current is available through the extension telephone.
If a second extension telephone is subsequently placed in the off-hook condition, the line voltage available to the Zener diode of the privacy circuit connected to the second extension telephone will be insufficient to drive the Zener diode into reverse saturation and the SCR of the privacy circuit will not be fired. As such, the second extension telephone will not gain access to the telephone line.
The primary drawback of the MacIntosh privacy circuit is that it requires the use of a separate privacy circuit for each extension telephone. If any extension telephone does not include a privacy circuit, a person picking up that extension telephone will be permitted to eavesdrop on the conversation of the party already using another extension telephone at that residence without the knowledge of the first user, although his extension telephone is equipped with the MacIntosh privacy circuit.
U.S. Pat. No. 4,136,266 to Cohen et al. discloses a privacy circuit which need only be connected in series with that extension telephone which is to be protected. This privacy circuit protects the fact that an extension telephone has been placed in the off-hook condition by detecting "click" signals which are placed on the telephone line whenever any extension telephone is placed either in the off-hook condition or, if it is already in the off-hook condition, when it is placed in the on-hook condition (defined herein as that condition wherein the hook switch of the telephone is in the lower or depressed position).
The "click" signals are defined in Cohen et al. as having distinctive acoustic and electrical properties which can be detected as a function of the frequency of the signal and the voltage level thereof. The Cohen et al. patent provides complex detection circuitry for detecting these click signals and generates an audible and/or visual alarm whenever these signals are detected.
In order to detect these signals, the privacy circuit of Cohen et al. relies on a tuned detection circuit which must be tuned to the specific frequency of the clicks. Since this frequency will vary from telephone to telephone, it requires a manual adjustment of a variable resistor forming part of the tuned circuit. This manual adjustment must be made by the user of the privacy circuit for each individual installation.